1. Field of the Invention
The invention relates to improvements in supply and distribution of gas to and within fluidized bed reactors in which fine particles of a material to be processed or to be made to react are held in suspension by an ascending stream of gas.
2. Description of the Prior Art
A common means for distribution of fluidizing gas in a reactor comprises a plate located at the base of the reactor through which an ascending current of fluidizing gas passes, and on which base the particles rest when the reactor is shut down. Alternatively, state-of-the-art reactors may have a dished or flat bottom with rows of pipe grids in a horizontal plane spaced some distance above the bottom for distributor of the fluidizing gas. The pipes may be arranged in many configurations such as parallel rows, concentric rings, or an outer ring with radial spokes resembling a wagon wheel. Holes are drilled in the pipes in a regular pattern for distribution of the gas within the bed; such holes may be oriented upward, downward, or horizontal.
The distributing plate, also called a grid, divides the reactor into two chambers namely; a first upper chamber constituting the treatment chamber into which the material in particulate form to be treated or to be made to react is introduced, and a second lower chamber called a "wind box" which is connected to the source of gaseous fluidizing agent. In some of these reactors the grid is in the form of a plate of varying thickness usually of a refractory material, perforated with a multitude of holes of relatively small section through which the ascending stream of fluidizing gas passes. In some reactors, such multitude of holes are replaced by a much smaller number of cavities of much greater section, cutting through the grid and placing the wind box in communication with the treatment chamber. Dual perforated plates may be spaced close together with the holes staggered so as to prevent direct "line-of-sight" connection between the two chambers. Alternatively, the holes in the plate or plates may be equipped with caps, bubble caps, risers and tuyers or distributors, ball-check valves; or the plate may be made of sintered porous plates. In other reactors, the cavities are joined to a common source of fluidizing agent by means of gas supply ducts independent of one another.
Usually, the cross-sections of the holes or of the cavities are greater than the size of the particles constituting the material to be processed or to be reacted. When the speed of the ascending stream of fluidizing gas is inadequate or nonexistant, the particles which are not in a state of fluidization flow downwardly into or through the holes or the cavities and accumulate either in the wind box or in the ducts supplying fluidizing agent to the cavities. This may lead to the clogging of the ducts, which may necessitate either the cleaning of the ducts, or their replacement. Sometimes the presence of the solids in the fluidizing gas stream outside of the treatment chamber can result in undesired chemical reactions or decomposition resulting in poor yields, undesired by-products and impurities, or even hazardous or unsafe operating conditions.
To prevent this undesirable flow, reactor grids fitted with a wind box have been provided with fixed or stationary, even retractable, valves, arranged either over or below the holes or the cavities, and intended to seal the said holes or the cavities at the proper times . These valves are not easily fitted to grids whose cavities are supplied with fluidizing gas independently of each other. The utilization of dual staggered-hole plate grids, hole caps, risers and tuyers and pipe grids with downwardly directed holes are all attempts to prevent such back flow of solids but which are only partially effective because semi-aerated solid particles negotiate the obstructions in these devices at the time of fluidizing gas shut-off, ball-check valves act as small grinding mills and reduce the particle size of the solid particles resulting in their loss through recovery equipment and resulting atmospheric pollution or downstream equipment plugging sintered porous plates prevent solids from weeping but are subject to pluggage by particulates in the fluidizing gas, and are difficult to install, seal, and replace.
In some fluid-bed processes, solids which are not fluidized or kept suspended are subject to hot spots, caking or production of off-grade products. Many distributors are incapable of placing all solids in suspension. Solids resting on plate distributors between cavities, or between hole caps, risers and tuyers below pipe distributors may be largely stagnant.
An object of the present invention is to provide an apparatus for supplying gas to a fluidized bed reactor, avoiding the above-mentioned disadvantages by eliminating passages which can be blocked, or plugged while preventing weepage of the solids back to the windbox, eliminating stagnant, non-fluidized solids areas, and reducing solids attrition to that caused by gas-solids interaction.
Another object is to reduce distributor assembly corrosion and catalyst bridging problems in reactor particle distributors.